The present invention relates to a tractor feeder for feeding continuous printing paper by driving paper feed pins which are engaged to paper feeding holes provided along both the side fringes of the continuous printing paper.
Continuous printing paper for use in a printer, for instance serving as an output device for a computer, is generally provided with paper feeding holes or perforations at equal intervals along the fringes on either side of the printing paper. A tractor feeder having an endless belt provided with a plurality of pins projecting therefrom is known as one of the paper feeding devices for use in printers which employ such continuous printing paper. According to a tractor feeder of this type, the endless belt is stretched between a drive sprocket and a guide portion of the frame provided opposite to the drive sprocket so that the belt may be driven by the drive sprocket.
To achieve proper paper feeding action, the motion of the belt must be proper. Therefore, the frame of a tractor feeder generally defines guide grooves at its upper and lower surfaces for the purpose of prohibiting the vertical and lateral deviation of the belt from its prescribed path, and the guide portion provided opposite to the drive sprocket is provided with a guide member which forms a semi-circular path in continuation from the guide grooves and is normally integral with the frame.
If there is any excessive slack in the endless belt, the engagement between the sprocket and the endless belt may not be assured. If the tension of the endless belt is excessive, the frictional resistance between the various moving parts of the tractor feeder and between the endless belt and the tractor feeder may increase and the durability of the system may be impaired.
U.S. Pat. Nos. 3,825,162 and 4,129,239 show tractor feeders which have freely rotatable but otherwise fixed guide members. If the position of the guide member is fixed, the tension of the endless belt is strongly affected by the deviation of the circumferential length of the endless belt from a certain standard length and exact tolerance control for the endless belt is necessary for accurate control of the tension of the endless belt.
U.S. Pat. No. 4,457,463 discloses a D-shaped shoe which is biased by a spring member towards the endless belt and applies a certain tension to the endless belt. U.S. Pat. No. 4,469,263 discloses spring biasing members which, integrally provided with spring leg portions and placed on the bottom surfaces of the grooves defining straight paths for the endless belt, bias the endless belt and apply certain tension thereto. According to this technique of using a spring member for applying tension to the endless belt, since the endless belt is always under a certain tension which is required to take away the slack from the endless belt, the frictional resistance which the endless belt receives from the tractor feeder can not be reduced to a very low level.
U.S. Pat. No. 3,930,601 discloses a tractor feeder having an idler molding having bifurcated legs. According to this tractor feeder, a pair of bolts are passed through elongated holes provided in the frame and the holes in the idler molding so that the idler molding may be fixed by fastening this bolt after adjusting the position of the idler molding. Thus, the endless belt may be put free from slack and no substantial residual tension will be produced in the endless belt, but the adjustment is not simple since the bolts must be fastened while the idler molding is securely held in a desired position for instance by hand.